1. Technical Field
The present invention relates to a process and apparatus for embossing material with precise detail, and more particularly, to a process and apparatus for making products of thermoplastic material having surfaces with precision microstructures, as defined below. It also pertains to a tool and method of making a tool for such embossing.
2. Background Art
Processes and apparatus for embossing precision optical patterns such as microcubes, in a resinous sheet or laminate, is well known as referenced in U.S. Pat. Nos. 4,486,363; 4,478,769; 4,601,861; 5,213,872; and 6,015,214, which patents are all incorporated herein by reference. In the production of such synthetic resin optical sheeting, highly precise embossing (generally exceeding the capabilities of the current micromolding processing techniques for synthetic resins), is required because the geometric accuracy of the optical elements determines its optical performance. The above referenced patents disclose in particular methods and apparatus for continuously embossing a repeating retro-reflective pattern of fine or precise detail on one surface of a transparent thermoplastic material film to form the surface of the film into the desired microstructure pattern.
Besides precision optical sheeting, various other applications have been developed requiring the formation of highly precise shapes and structures in resinous film. Such applications include (in addition to optical applications) micro-fluidic, micro-electrical, micro-acoustic, and micro-mechanical applications. Such applications require the embossing of thermoplastic material to provide precisely formed functional geometric elements, or arrays of such functional geometric elements on the film surface.
These geometric elements, or precision microstructures, are defined by any or all of the following characteristics: precise embossing depths; flat surfaces with precise angular orientation; fine surface smoothness; sharp angular features with a very small radius of curvature; and precise dimensions of the elements and/or precise separation of the elements, within the plane of the film. The precise nature of the formed surface is critical to the functional attributes of the formed products, whether used for microcubes or other optical features; or as channels for microfluidics, or in fuel cells; or for accurate dimensions, flatness and spacing when providing a surface for holding nanoblocks in Fluidic Self Assembly (FSI) techniques; or imparting a microtextured surface that is not optically smooth within an array that includes, or excludes additional microarchitecture.
U.S. patents describing some uses of precise microstructures include: U.S. Pat. Nos. 4,486,363; 6,015,214 (microcubes); U.S. Pat. Nos. 5,783,856; 6,238,538 (microfluidics); and U.S. Pat. No. 6,274,508 (FSA).
As described in some of the above mentioned patents, such as U.S. Pat. Nos. 4,486,363, 4,601,861, and 4,478,769, embossed microstructure film may be made on a machine that includes two supply reels, one containing an unprocessed film of thermoplastic material, such as acrylic or polycarbonate, or even vinyl, and the other containing a transparent and optically smooth plastic carrier film such as Mylar, which should not melt or degrade during the embossing process. These films are fed to and pressed against a heated embossing tool which may take the form of a thin endless flexible metal belt. The belt creates the desired embossed pattern on one surface of the thermoplastic film, and the carrier film makes the other surface of the thermoplastic film optically smooth.
The belt moves around two rollers which advance the belt at a predetermined linear controlled speed or rate. One of the rollers is heated and the other roller is cooled. An additional cooling station, e.g. one that blows cool air, may be provided between the two rollers. Pressure rollers are arranged about a portion of the circumference of the heated roller. Embossing occurs on the web as it and the tool pass around the heated roller and while pressure is applied by one or more pressure rollers causing the film to be melted and pressed onto the tool. The embossed film, (which may have been laminated to other films during the embossing process), is cooled, monitored for quality and then moved to a storage winder. At some point in the process, the Mylar film may be stripped away from the embossed film.
The prior apparatus and process work well to produce rolls of film that are effectively 48″ (122 cm) wide (52″/132 cm at salvage), but such equipment and processes have several inherent disadvantages. First, the process speed (and thus the volume of material) is limited by the time needed to heat, mold and freeze the film. Also, the pressure surface area and thus the time available to provide adequate pressure by the pressure rollers, impose certain special constraints; and then cooling the material. Finally the formation of some embossed surfaces while the tool is in a curved condition requires complex modification of the geometry of the tool surface, because the thermoplastic elements are formed while on a curved surface but generally used later while on a flat surface.
One earlier prior device for forming microcubes while in a planar condition is illustrated in U.S. Pat. No. 4,332,847, and involves indexing of small (9″×9″ or 22.86 cm×22.86 cm) individual molds at a relatively slow speed (See Col. 11, lines 31-68). That process is not commercially practical because of its perceived inability to accurately reproduce microstructures because of indexing mold movement and the relatively small volume (caused by mold size) and speed. Also, the equipment and process is non-continuous.
It is apparent that there is a need for equipment and processes that permit a larger volume of precision microstructure material to be produced in a given time, and using tools that may heat, emboss and cool the film while in a planar condition.
Continuous press machines have been used in certain industries and provide some of these features, but to applicants' knowledge, had not previously been modified nor used to produce previous microstructures. These machines include double band presses which have continuous flat beds with two endless bands or belts, usually steel, running above and below the product and around pairs of upper and lower drums or rollers. An advantage of such presses is the mainly uniform pressure which can be provided over a large area. These machines form a pressure or reaction zone between the two belts and have the advantage that pressure is applied to a product when it is flat rather than when it is curved. The double band press also allows pressure to be adjusted over a wide range and the same is true of temperature variability. Dwell time or time under pressure also is controllable for a given press by varying the production speed or rate, as is capacity, which may be changed by varying speed and/or length and/or width of the press. The bands may have highly smooth surfaces, or alternatively may in some cases have macrostructured surfaces for forming desired structure in the product passing through the press. However, such presses, without modifications, are neither designed for nor capable of embossing precise microstructures on the material passing through the press.
In use, the product raw material is fed between the two belts and drawn into the press at a constant speed. The belts heat and press the material in a direction that is normal to its motion, in a relatively long flat plane. Of course, friction is substantial on the product, and double band presses overcome this by several systems. One system is the gliding press, where pressure-heating plates are covered with low-friction material such as polytetrafluorethylene and lubricating oil. Another is the roller bed press, where rollers are placed between the stationary and moving parts of the press. The roller press is sometimes associated with the term “isochoric”.
The third type of press is the fluid or air cushion press, which uses a fluid cushion of oil or air to reduce friction between the belt and the rest of the machine. It was conceived by applicants that this type of press may be suitable for precision embossing of microstructures. The fluid cushion press is sometimes associated with the term “isobaric.” Isobaric presses currently available may operate to about 1000 psi (6.89 MPa), and at temperatures up to 662° F. (350° C.). Air has the advantage of providing a more uniform pressure distribution over the entire width and length of the press in the reaction zone. With either thick or thin raw material, or substrate, heat is transferred from heated pressure plates to the belts and then to the film or substrate.
In an isobaric press, heat generally does not come from rollers or drums; rather the fluid, e.g. air, provides it. The fluid transfers heat to the steel belts which in turn transfer the heat to the material pressing through the press. This provides one advantage over prior art forms of embossing equipment—the ability to heat the film to be embossed from both sides.
Another advantage of the double band press is that the raw material may be heated first and then cooled, while the product is maintained under pressure. Heating and cooling elements may be separately located one after the other in line behind the belts. The steel press belts are first heated and then cooled, thereby efficiently heating and then cooling the material in the reaction zone and all while under pressure.
Continuous press machines fitting the general description provided hereinabove are sold by Hymmen GmbH of Bielefeld, Germany (U.S. office: Hymmen International, Inc. of Duluth, Ga.) as models ISR and HPL. These are double belt presses and also appear under such trademarks as ISOPRESS and ISOROLL. Typically they have been used to produce relatively thick laminates, primarily for the furniture industry.
U.S. Pat. No. 4,844,766 to Kurt Held discloses a double band pressure embossing press for fabrication of thermoplastic webs from thermoplastic resinous material heated to a processing temperature. The '766 patent discloses that the thermoplastic webs may be smoothed and calibrated, and may be provided with relatively coarse surface structures such as knobs, ornaments, corrugations, etc. by providing the press band with structured surfaces. The thermoplastic web first passes through a heated zone on the press, then a cooling zone at which smoothing of the web surface is terminated. However, the '766 patent does not disclose a method or apparatus for providing thermoplastic webs with precision microstructured features, as achieved by the present invention. Moreover, neither the Hymmen presses nor the Held patent disclose several other important aspects of embodiments of the invention which are related to the the embossing tool. Preferably, the embossing tool will be made of a different material than the steel belts heretofore used, thus allowing for ease in the manufacture of precision microstructure features.
Even though double band relatively coarse embossing of thermoplastic materials is known (“course” to the point that structures produced are not essential for the products performance), in the prior processes the geometric tolerance from the embossing belt or tool to the film was not critical for overall product performance. Significant improvements are still needed to increase manufacturing precision and efficiency, improve quality and lower the cost of producing finished products.
Thus an object of the present invention is to provide a process and apparatus for efficiently, effectively, and inexpensively embossing thermoplastic material with precise microstructure detail.